Fix handling of virtual exit node in PostDomTree (#53739)

This is an alternative to #53642

The `dom_edges()` for an exit block in the CFG are empty when computing
the PostDomTree so the loop below this may not actually run. In that
case, the right semidominator is the ancestor from the DFSTree, which is
the "virtual" -1 block.

This resolves half of the issue in
#53613:
```julia
julia> let code = Any[
               # block 1
               GotoIfNot(Argument(2), 3),
               # block 2
               ReturnNode(Argument(3)),
               # block 3 (we should visit this block)
               Expr(:call, throw, "potential throw"),
               ReturnNode(), # unreachable
           ]
           ir = make_ircode(code; slottypes=Any[Any,Bool,Bool])
           visited = BitSet()
           @test !Core.Compiler.visit_conditional_successors(CC.LazyPostDomtree(ir), ir, #=bb=#1) do succ::Int
               push!(visited, succ)
               return false
           end
           @test 2 ∈ visited
           @test 3 ∈ visited
       end
Test Passed
```

This needs some tests (esp. since I don't think we have any DomTree
tests at all right now), but otherwise should be good to go.

base/compiler/ssair/domtree.jl

@@ -345,10 +345,7 @@ function SNCA!(domtree::GenericDomTree{IsPostDom}, blocks::Vector{BasicBlock}, m
     ancestors = copy(D.to_parent_pre)
     relevant_blocks = IsPostDom ? (1:max_pre) : (2:max_pre)
     for w::PreNumber in reverse(relevant_blocks)
-        # LLVM initializes this to the parent, the paper initializes this to
-        # `w`, but it doesn't really matter (the parent is a predecessor, so at
-        # worst we'll discover it below). Save a memory reference here.
-        semi_w = typemax(PreNumber)
+        semi_w = ancestors[w]
         last_linked = PreNumber(w + 1)
         for v ∈ dom_edges(domtree, blocks, D.from_pre[w])
             # For the purpose of the domtree, ignore virtual predecessors into